The Influence of Machining Parameters on Surface Roughness in Double Tool Turning Process
6.3 Results and Discussion
6.3.2 Effect of Feed on Surface Roughness
The thermogram (Figure 6.4) displays the brittle chips flowing away from the cutting zone causing lesser heat generation in the secondary deformation zone. The details of brittle chip morphology of grey cast iron are mentioned in Section 5.3.5 of Chapter 5. The chips exhibited fracture and no thermal effect was observed. On the other hand for AISI 1050 steel workpiece material, the continuous chips are produced which in turn generate more heat due to sliding of chips over the rake face of the cutting tool. Unlike the front cutting tool as the rear cutting tool is inverted, its cutting zone is only partially visible. Apart from this, the cutting tool vibration also plays a significant role in determining the average surface roughness of the machined work piece. Increase in cutting speed results in decreased tool vibration. It was already reported in Section 4.5 of Chapter 4 that when the cutting speed is increased from 75 m/min to 185 m/min the amplitude of tool vibration along the cutting direction is reduced. This in turn reduces the surface roughness, thereby improving the surface finish and stability of the double tool turning process. Dimla (2004) also observed a reduction in amplitude of cutting tool vibration when the cutting speed was increased from 100 m/min to 300 m/min while machining steel with a carbide tool. Thus it can be summarised that the double tool turning process resembles conventional turning process where the surface roughness decreased with the increase in cutting speed and increased with the decrease in cutting speed.
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ductile materials like steel. On increasing the feed from 0.04 mm/rev to 0.12 mm/rev material is removed by shearing action, thereby reducing the average surface roughness. The burnishing action of the cutting tool over the workpiece might also lead to decreased average surface roughness. Thereafter an increased average surface roughness is obtained up to 0.24 mm/rev feed.
Figure 6.5. Variation of surface roughness with feed for AISI 1050 steel for 1 mm depth of cut and 2 mm tool separation distance (a) For the cutting speeds ranging from 75 to 225 m/min (b) For the cutting speeds ranging from 100 to 250 m/min
Risbood et al. (2003) observed a similar trend for mild steel; the average surface roughness got minimised by 68% when the feed was increased from 0.05 mm/rev to 0.14 mm/rev and on further increase of feed to 0.36 mm/rev the average surface roughness increased. Gokkaya and Nalbant (2007) observed an increase of surface roughness from 1.616 µm to 6.833 µm when the feed was increased from 0.15 mm/rev to 0.35 mm/rev while turning AISI 1030 steel with coated carbide tool.
Higher rate of deformation of the work material and cutting edge feed marks are the reasons behind higher surface roughness at higher feed. It is noteworthy to mention here that cutting edge feed marks depends on the nose radius of the cutting tool also, which is 0.8 mm in the present investigation. The feed marks for three different feeds
of AISI 1050 steel are unfolded in Figure 6.6, Figure 6.7 and Figure 6.8. It can be seen that the average surface roughness is lower at a feed of 0.12 mm/rev when compared to 0.04 mm/rev and 0.24 mm/rev. The three dimensional topographies reveal the peak to valley height of the scanned areas for various feed. This gives an apprehension of the maximum surface roughness of the machined surface. The Table 6.2 provides the peak to valley height and average surface roughness at different feeds of AISI 1050 steel work material. The machining parameters are 75 m/min cutting speed, 1 mm depth of cut for both front cutting tool and rear cutting tool and 2 mm tool separation distance.
Figure 6.6. Three dimensional topography of AISI 1050 steel at 0.04 mm/rev feed (75 m/min cutting speed, 1 mm depth of cut 2 mm tool separation distance)
It was identified that the peak to valley height decreased by 37% when the feed increased from 0.04 mm/rev to 0.12 mm/rev. For further increase of feed to 0.24 mm/rev, it increased by 89%. It can be envisaged that for the same cutting conditions the average surface roughness decreased by 32% and then increased by 76%. The three dimensional surface roughness profile of Figure 6.6 has a predominance of green colour. This shows that the existing grooves are of uniform depth. It can also be noted that the waviness of the surface is minimal.
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Table 6.2. Variation of peak to valley height and average surface roughness with feed for AISI 1050 steel
Feed (mm/rev) Peak to valley height Rmax (µm)
Average surface roughness Ra (µm)
0.04 19.8 3.75
0.12 12.5 2.55
0.24 23.6 4.5
Figure 6.7. Three dimensional topography of AISI 1050 steel at 0.12 mm/rev feed (75 m/min cutting speed, 1 mm depth of cut 2 mm tool separation distance)
In Figure 6.7, substantial amount of blue region is present along with slight red and green regions. The blue region represents a minimum height of peak to valley, hence the surface roughness is lesser at this feed. In the Figure 6.8 apart from the grooves along the cutting direction, some micro grooves are also observed along the direction perpendicular to it. It is due to the ploughing action of the cutting tool. This deteriorates the surface finish resulting in higher surface roughness. Apart from this, pertinent red region showing the maximum peak to valley height also exists. Hence the average surface roughness is maximum at higher feed. It is to be noted that the lowest and highest average surface roughness value of 1.32 µm and 4.5 µm for AISI
1050 steel is obtained at cutting speed of 250 m/min and 75 m/min for the feeds of 0.12 mm/rev and 0.24 mm/rev respectively.
Figure 6.8. Three dimensional topography of AISI 1050 steel at 0.24 mm/rev feed (75 m/min cutting speed, 1 mm depth of cut 2 mm tool separation distance)
Figure 6.9. Variation of surface roughness with feed for grey cast iron for 1 mm depth of cut and 2 mm tool separation distance (a) For cutting speed ranging from 75 m/min to 225 m/min (b) For cutting speed ranging from 100 to 250 m/min
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Figure 6.9 (a) and (b) exhibits the variation of surface roughness of grey cast iron with the change in feed for various cutting speeds. Previous researchers had suggested an increase of surface roughness with the increase in feed (Ramesh et al., 2012; Upadhyay et al., 2013). It is seen that the average surface roughness increased from 1.40 µm to 2.87 µm with the increase in feed from 0.04 mm/rev to 0.24 mm/rev for the investigated cutting speeds. This coincides with the results of Gunay and Yucel (2013), it was observed that the surface roughness increased by 42% when the feed was increased from 0.05 mm/rev to 0.1 mm/rev while turning white cast iron with a ceramic tool. In the current investigation the lowest and highest surface roughness value of 1.06 µm and 3.5 µm was obtained for a feed of 0.04 mm/rev and 0.24 mm/rev at a cutting speed of 250 m/min and 75 m/min respectively. At this juncture it can be well emphasized that combination of low feed with high cutting speed will produce lower surface roughness for grey cast iron and vice versa. On similar lines the work of Xavior and Adhithan (2009) revealed that the surface roughness increased from 1.91 µm to 2.68 µm when the feed was increased from 0.2 mm/rev to 0.28 mm/rev, while turning austenitic steel with carbide tool using different cutting fluids. It is to be noted that in the present work, feed of both the front and rear cutting tools are equal as both are mounted on the same carriage. Thus the investigation reveals that for grey cast iron the average surface roughness increases with the increase in feed. However AISI 1050 steel the average surface roughness decreases when the feed was increased from 0.04 mm/rev to 0.12 mm/rev and on further increase of feed from 0.12 mm/rev to 0.24 mm/rev the average surface roughness increased.